The invention relates to receiving systems and methods for TV audio processing, and more particularly, to a receiving system complying with multiple audio standards and related method thereof. Audio stereo signal standards for TV broadcasts include Zweikanalton, also known as German Stereo or A2 Stereo, Near Instantaneous Companded Audio Multiplex (NICAM), and Multichannel Television Sound (MTS), which is also known at the BTSC standard. Standards such as Zweikanalton and NICAM rely on two separate FM carriers. The second FM carrier of Zweikanalton, for example, is transmitted on a frequency 242 kHz higher than the main FM carrier. It can carry either a completely separate audio program, or be used for stereo sound transmission. In the latter case, the first FM carrier carries L+R for compatibility, while the second carrier carries 2*R. The second carrier also contains a control tone to indicate whether the transmission is stereo or dual sound. Absence of this tone is interpreted as a monaural transmission. A receiver capable of receiving and processing signals of such two-carrier system has two sets of audio signal processing hardware for processing signals carried by the two carrier concurrently.
The MTS signal is transmitted at a designated carrier frequency as part of the composite broadcast television video. The MTS signal comprises two or more channels, a first channel, which is the main channel formed as the sum of the left and right audio signals, and can be detected by both monophonic TV receivers and stereo receivers. A second channel is formed as the difference between the left and right audio signals, and is detectable only by stereo receivers. A SAP channel is used to provide a Supplemental Audio Program (SAP) such as a second language, for example Chinese.
The MTS receiver performs two steps of demodulation, first FM (frequency modulation) demodulation, follows by AM (amplitude modulation) demodulation for stereo audio input, or FM demodulation for dual audio input. For example, an FM demodulator circuit demodulates a stereo signal and removes the FM carrier to output a composite audio signal. The composite audio signal is then coupled to a signal processing circuit that separates the various audio channels with AM demodulation. Corresponding audio signals L and R can thus be output for reproduction by the television speakers.
Vendors can hold the edge over their rivals if a solution complying with different audio standards is provided. An instinct way to satisfy different TV audio standards, say MTS and Zweikanalton, is to equip receivers with parallel audio signal processing hardware. When receiving audio channels from a Zweikanalton system, the receiver demodulates signals carried by two carriers with two audio signal processing hardware concurrently, and when receiving audio channels from a MTS system, only one signal processing hardware is occupied.
Please refer to the receiving system illustrated in FIG. 1, which is capable of receiving audio channels from a Zweikanalton system or a MTS system. The two sets of audio signal processing hardware of the receiving system in FIG. 1 operate to process audio signals carried by two carriers concurrently if it is working with the NICAM system. One of the signal processing hardware is unused if the receiving system is working with the MTS system.